The formation and retention of charges of static electricity on the surface of most plastics is well known. Plastic materials have a significant tendency to accumulate static electrical charges due to low electrical conductivity. The presence of static electrical charges on sheets of thermoplastic film, for example, can cause the sheets to adhere to one another thus making their separation for further processing more difficult. Moreover, the presence of static electrical charges causes dust to adhere to items packaged in a plastic bag, for example, which may negate any sales appeal. Static build up in containers for microprocessors and photographic film presents problems as well as in environments containing explosive dust or fumes.
The increasing complexity and sensitivity of microelectronic devices makes the control of static discharge of particular concern to the electronic industry. Even a low voltage discharge can cause severe damage to sensitive devices. The need to control static charge buildup and dissipation often requires the total assembly environment to be constructed of partially conductive materials. It also may require electrostatic protective packages, tote boxes, casings, and covers be made from conductive polymeric materials to store, ship, protect, or support electrical devices and equipment.
The prevention of the buildup of static electrical charges which accumulate on plastics during manufacturing or use has been prevented by the use of various electrostatic dissipative (ESD) materials. These materials can be applied as a coating which may be sprayed or dip coated on the article after manufacture although this method usually results in a temporary solution. Alternatively these materials can be incorporated into the polymer during processing thereby providing a greater measure of permanence. However, the incorporation of low molecular weight electrostatic dissipative materials (anti-static agents) into the various polymers has its own limitations. For example, during the hot temperatures required during conventional processing many of the antistatic agents cannot withstand high temperatures and are damaged or destroyed, thereby being rendered useless with respect to their ESD properties. Also, many of the higher molecular weight ESD agents are not miscible with the base polymers employed, and if the refractive indices differ by more than about 0.02, there can be a substantial opacity in the composition. In an immiscible polymer blend where the particle size of a dispersed phase is greater than 0.1 micron, the smaller the difference in the refractive indices between the additives and the base polymer the greater the clarity of the article made from the mixture.
A large number of anti-static agents are also either cationic or anionic. These tend to cause the degradation of plastics, particularly PVC, and result in discoloration or loss of physical properties. Low molecular weight anti-static agents possess undesirable lubricating properties and are difficult to incorporate into the polymer. Incorporation of the low molecular weight anti-static agents into the polymers often will reduce the moldability of the base plastic because the antistatic agents can move to the surface of the plastic during processing and frequently deposit a coating on the surface, producing excessive marring or poor surface finish on the articles of manufacture. In severe cases, the surface of the article of manufacture becomes quite oily or marbleized. Additionally, the low molecular weight ESD agents tend to lack permanence and lose their ESD capability due to evaporation. There can be undesirable odors, stress cracking or crazing can develop on the surface of an article in contact with the article of manufacture.
One of the known lower molecular weight antistatic agents is a homopolymer or copolymer oligomer of ethylene oxide. Generally, use of the lower molecular weight polymers of ethylene oxide or polyethers as antistatic agents are limited by the above-mentioned problems relative to lubricity, surface problems, or less effective ESD properties. Further, these low molecular weight polymers can be easily extracted or abraded from the base polymer thereby relinquishing any electrostatic dissipative properties. Polyetheresteramide block copolymers (PEA) have higher molecular weight and thus would have better permanence of properties, however with respect to polyvinyl chloride polymers, there are problems pertaining to compatibility and impact strength. With some polyetheresteramides there are incidences of lack of uniform dispersion within the PVC matrix. There are problems relating to the reduction of moduli and impact strength of mixtures of PEA and polyvinyl chloride.
The high polymer electrostatic dissipative agents in general derived from polymers of ethylene oxide or a derivative thereof like propylene oxide, epichlorohydrin, glycidyl ethers and the like having high haze values and thus lack transparency desired for some end uses. This is especially true for combinations of PVC and certain polyetheresteramides.
Antistatic properties for thermoplastic polyvinyl chloride resins which are suitable for injection molding has increased commercial importance, however these resins have inherent rheological characteristics which limit the number of ESD additives which can be successfully melt mixed to form a uniform blend of commercially acceptable appearance and physical properties. This invention is directed to a solution for this problem.
In the modification of vinyl thermoplastics such as polyvinyl chloride, there must be sufficient mechanical compatibility between the antistatic polymer and the vinyl polymer in order that the blend will have sufficient tensile strength and impact resistance, however in order for achieving the desired degree of ESD performance a discrete dispersed phase of ESD agent is necessary. This dictates that a certain degree of incompatibility must exist between the matrix polymer phase polyvinyl chloride, and the dispersed phase ESD agent which results in poor impact strength. It has been found that conventional impact modifiers in combination with polyetheresteramide and polyvinyl chloride interfere with static decay rate and volume resistivity. This invention is aimed at a solution to this problem.
Certain polyetheresteramide block copolymers having acceptable static dissipative properties are non-rigid owing to the proportion of polyether segment in the block copolymer. These non-rigid materials may be blendable with rigid PVC thermoplastic resins but there is presented a problem of reduction in moduli and heat distortion temperature which limits the commercial applications for such a compound. This invention addresses a solution to this problem.